This is a DMfit 12mm to 12mm shut off valve tap with push fit connections. This fitting is specified to be used with reverse osmosis systems, filters.

Also compatible with the John Guest 12mm.

There are numerous material types this part will work in conjunction with. These include tubing (as well as flexible tubing like silicone and Tygon®, although soft tube supports will be needed), soft metals (copper, brass, mild steel) and semi-rigid plastics (nylon, polyethylene, polypropylene). 

DM-fit 12mm Pushfit Shut Off Valve Connection